1. Field of the Invention
The present invention relates to an optical multiplexer; and more particularly, to a multi-wavelength, bi-directional optical multiplexer useful in an optical fiber data transmission system.
2. Description of the Prior Art
Transmission of data by optical fiber waveguides, also called fiber optics or optical fibers, has become ubiquitous in the telecommunications and computer industries. Digital information in an electronic system is converted into a series of pulses of light generated by laser diodes (LD's) or light emitting diodes (LED's), which are injected into long fibers of glass or polymeric materials. The fibers are capable of propagating the light with extremely low losses and acceptably low dispersion, whereby information embodied in the modulation pattern may be conveyed. The light that emerges from the other end of the fiber can be detected and reconverted into electronic signals that faithfully reproduce the original signal.
Fiber optic communication has a number of advantages over traditional transmission means such as hard-wired coaxial and twisted pair cables and lower frequency electromagnetic broadcasting of radio and television signals. Foremost is the much larger bandwidth available. In addition, existing infrastructure such as cable ducts, utility poles, and the like presently used by telecommunications and cable television (CATV) companies can be upgraded with relatively little disruption and moderate cost by substituting optical fiber cable for existing copper wire. Thus, dramatic increases in bandwidth needed to accommodate the needs of an information-based, Internet-driven society and commerce can be obtained with comparatively little disruption.
While fiber optic-based communications has become commonplace for trunk lines in the telephone system, e.g. for long-distance telephone and data transmission and linking central offices, the final connection of these services to homes, businesses, and other service users is still largely done with conventional electrical wiring. In addition, a large fraction of households subscribe to CATV services that distribute plural television channels via coaxial cable. Some CATV providers also offer data connections, e.g. to the Internet. While coaxial service provides a much wider bandwidth than ordinary twisted-pair wires, many customers crave even greater bandwidth. A distribution system implemented with fiber optic service potentially can provide the enough bandwidth for: (i) high data rate communications, e.g. via the Internet; and (ii) enhanced CATV video services, e.g. movies on demand to complement conventional broadcasting.
Ideally, a system would employ a single optical fiber to serve an optical network unit (ONU) located in or near each customer's premises. The fiber would bi-directionally carry a full range of the communication forms that customers desire, including telephone, data, and video. In addition, such a system should be inexpensive, flexible, and compatible with existing communications infrastructures and protocols to the greatest possible extent.
The effective bandwidth of a given optical fiber can further be expanded by wavelength multiplexing, wherein plural communications channels are carried as modulated light of different wavelengths simultaneously propagating in a single fiber. Bi-directional or full-duplex transmission is easily handled in a multiplexed system by assigning different wavelength channels simultaneously usable for upstream and downstream transmission. However, the practical implementation of multiplexed systems depends on the availability of suitable multiplexing converters that separate the different wavelengths and translate optical signals to and from corresponding electrical impulses.
Protocols and equipment parameters for telecommunications systems are frequently specified by relevant government and international agencies, such as the International Telecommunications Union (ITU), and by recognized technical societies that promulgate standards, such as the Institute of Electrical and Electronics Engineers (IEEE). Several current ITU and IEEE standards, including ITU-T Standards G983.1 and G984.2 and IEEE Standards 802.3ah, relate to multiplexed optical fiber communications. Each of the aforesaid ITU and IEEE standards is hereby incorporated in the entirety by reference thereto. The standards all entail multiplexing light signals bearing both low-power, bidirectional digital data and high-power analog CATV video content. A wavelength of 1.55 μm is specified for the high-power analog video signal, with low-power digital data download to the user at 1.49 μm and digital data upload at 1.31 μm. These three wavelengths are readily propagated with low loss and dispersion in known single-mode optical fibers. While these protocols provide the services widely demanded by users, their implementation in a practical, cost-effective manner presents significant challenges, most notably the separation of the low and high power signals with adequate immunity to cross-talk. The power levels of the respective incoming signals are often 30 dB or more different. The allowable cross-talk level must therefore be at least about 10 dB below the sensitivity of the low-power receiver, which is often about −30 dBm. At that level, the low-power data is sufficiently immune to noise originating in the video signal. These standards provide for data communications at data rates that desirably are as high as 1.25 Gb/s or more in both upstream and downstream directions.
Furthermore, different customers for a multiplexed, combined video and data service have different needs and desires for service levels. In particular, it would be valuable to offer the video and data services either singly or in combination, since some customers would have little desire for both and accordingly would not be inclined to pay for the full range of features.